Microelectromechanical Systems (MEMS) devices are widely used in applications such as automotive, inertial guidance systems, and household appliances, protection systems for a variety of devices, and many other industrial, scientific, and engineering systems, such as in switches, pumps and motors. Such MEMS devices are used to sense a physical condition such as acceleration, pressure, or temperature, and to provide an electrical signal representative of the sensed physical condition.
MEMS devices may have parts moving mechanically under the influence of an external energy (acoustic, thermal, or optical), a temperature or voltage difference, or an external force or torque. Certain MEMS devices with a membrane, plate or beam can be used as inertial sensors.
In a MEMS device, the mechanically moving parts are sometimes fabricated together with the sensors and actuators in the process flow of an electronic integrated circuit (IC) on a semiconductor chip. Because of the moving and sensitive parts, MEMS devices may have a need for physical and atmospheric protection. Consequently, MEMS devices are surrounded by a housing or package, which shields the MEMS device against ambient and electrical disturbances, and against stress.
In the specific case of MEMS devices operating as accelerometers, for example, multiple devices may be fabricated over a single semiconductor wafer. After the devices are formed over the semiconductor wafer, a cap wafer is secured over the fabricated devices. After the substrate is singulated, the portions of the cap wafer covering each device become shields for the devices' moveable internal parts.
In addition to providing environmental protection, to provide electrical isolation and thereby allow for accurate operation of the MEMS device, the cap may also hold a given electrical potential. Generally, a wire or other conductive medium connects the cap to a cap terminal located in a base of the MEMS device. The desired cap potential is then applied to that terminal. Sometimes, however, the electrical connection between the cap terminal and the cap itself can become disconnected. The failure may be due to wire bond failure, for example. Unfortunately, in conventional MEMS devices, these failures can be difficult to detect and can result in a malfunctioning device.